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- Title
Simulations of the Surf Zone Eddy Field and Cross‐Shore Exchange on a Nonidealized Bathymetry.
- Authors
O'Dea, Annika; Kumar, Nirnimesh; Haller, Merrick C.
- Abstract
X‐band radar observations from the 2017 Inner Shelf Dynamics Experiment (ISDE) in central California show multiple persistent and pulsatory rip currents on a relatively straight coastline with alongshore‐varying bathymetry. Although past studies have assessed the characteristics of transient rip currents on alongshore uniform beaches, the relative balance of transient versus steady rip current behavior on nonuniform beaches in realistic wave conditions remains poorly understood. Here, a phase‐resolving Boussinesq‐type wave model (funwaveC) is used to assess the role of alongshore‐varying bathymetry and incident conditions in controlling mean and transient surf zone vorticity and velocity fields and their effect on surf zone exchange. The model simulates wave conditions chosen from the ISDE observations and utilizes both an alongshore‐varying bathymetry estimated from the ISDE radar observations and a uniform bathymetry. Results show that the variable bathymetry significantly increases the alongshore‐ and time‐averaged kinetic energy but that this increase is primarily due to the increase in the standing component resulting from mean circulation patterns, with only small changes in the transient component. A variable bathymetry also increases the spectral energy of surf zone vorticity and time‐averaged vorticity forcing at large spatial scales (>100 m). Wave directional spreading has a large impact on the alongshore‐ and time‐averaged enstrophy and on the spectral energy of surf zone vorticity and vorticity forcing at smaller spatial scales (<100 m). In the presence of a directionally spread wavefield, an alongshore‐varying bathymetry slightly increases the total exchange velocity but has little effect on its transient component. Plain Language Summary: Rip currents, offshore‐oriented currents generated by wave breaking in the surf zone, are the primary mechanism by which material such as sediment, pollution, and larvae are transported from the surf zone to the inner shelf. Although many rips form due to alongshore‐varying underwater topography (bathymetry) and are therefore fixed in space, they contain both a mean current field as well as a transient current field that fluctuates through time. Additionally, purely transient rip currents can form due to short‐crested wave breaking on uniform beaches. Although several studies on transient rips have been conducted on uniform beaches, the relative importance of transient versus steady rip current behavior on nonuniform beaches remains poorly understood. In this study, a computational wave model (funwaveC) is used to assess circulation and exchange due to rips on a nonuniform beach in a range of wave conditions. Results show that the presence of bathymetric variability significantly increases the kinetic energy within the surf zone (which describes the energy due to fluid motion) but only slightly increases the enstrophy (which describes the rotational properties of the fluid). The magnitude of the enstrophy was influenced primarily by wave directional spreading. Key Points: A Boussinesq wave model is used to analyze mean and transient vorticity and velocity dynamics on an alongshore‐varying versus uniform beachBathymetric variability strongly increases the total kinetic energy in the surf zone but has only a small impact on the total enstrophyDirectional spreading controls the spectral energy of surf zone vorticity at small scales, with bathymetry playing a role at larger scales
- Subjects
OCEANOGRAPHIC observations; OCEAN currents; OCEAN circulation; BATHYMETRY; MARINE geophysics
- Publication
Journal of Geophysical Research. Oceans, 2021, Vol 126, Issue 5, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016619